Corneal transplantation is the replacement of diseased corneal tissue with healthy corneal tissue. As a rule, it is not the entire cornea that is transplanted, but only a part of it (the size will vary).
In principle, medicine differentiates between replacement of the full thickness of the cornea (penetrating or perforating keratoplasty) and replacement of those partial layers (also called lamellar keratoplasty) of the cornea (lamellar keratoplasty) to which the pathological changes may be limited.
The human cornea has three different tasks in the eye: On the one hand, the cornea ensures that light rays can enter the eye and reach the retina without hindrance so we can see. And with it being curved, the cornea also has the necessary refractive power – together with the lens in the eye – to sharply reproduce the image of the outside world on the retina. And last but not least, the cornea is an integral part of the outer shell that protects the eyeball and gives it its shape.
Once the cornea is no longer able to adequately fulfil one or more of its functions, only the transplantation of a clear human cornea (corneal transplantation, or keratoplasty in medical circles) helps if no remedy can be provided by other means.
If the cornea becomes cloudy due to illness or injury, transplantation may be the last chance to protect you from complete blindness. Disease of the cornea often comes to light when you feel like you are peering “through frosted glass”, with everything very blurred.
In addition, things usually appear very blurred in the morning, an increased sensitivity to glare bothers you when driving in half-light or at night, you find it increasingly difficult to read, and feel increasingly restricted in your everyday life so end up complaining about loss of quality of life. Frequent new prescriptions for glasses or redness and burning of the eye (a bit like sand in the eye) can also be indications of cornea disease.
The most common causes that make corneal transplantation necessary are abnormal curvature or clouding of the cornea, usually due to swelling or scars.
Swelling occurs when the corneal endothelium (the inside of the cornea is covered by only one cell layer, the endothelium) fails. Scars on the cornea can occur as a result of injuries, burns or ulcers, among other things. Corneal transplantation may also be necessary in the event of severe inflammation of the cornea (keratitis), which also causes scarring or detachment of the cornea. This occurs, for example, after an infection of the cornea with herpes viruses (herpes keratitis). In addition, certain hereditary diseases and metabolic disorders may cause the cornea to become cloudy.
However, age-related or surgical procedures can also lead to corneal changes that require a transplant. Another possible cause is congenital corneal diseases.
Nowadays, there are modern devices that make it possible to measure the cornea accurately. Particularly important is the measurement of corneal curvature (medical term: topography) and corneal thickness (medical term: pachymetry). Particularly in the case of a keratoconus, this makes it possible to accurately estimate any damage that has already occurred and to monitor the course of the disease as best as possible (keratoconus is a pathological conical bulging of the cornea, which can progress in stages).
A high-resolution laser scan of the cornea makes it possible to precisely examine the inner pump cells (which make up the endothelium) of the cornea. Endothelial cells do not regenerate and cell density slowly decreases over the course of a person's life. If cell density falls below a certain threshold, the endothelium can no longer fulfil its function. Too much water flows in, and the cornea swells up and becomes cloudy.
In addition, the so-called slit lamp, a standard instrument in every practice, allows an ophthalmologist to thoroughly examine the cornea and also to assess the likely cause of the cornea clouding.
The ophthalmologist can also use a so-called Scheimpflug camera for diagnostic purposes, which generates a sectional image of the anterior segment of the eye at various levels. This can then be used to calculate a 3D model of the entire cornea.
The use of short-coherence light can help to create high-resolution images of the cornea. This so-called anterior segment OCT shows the individual layers of the cornea in detail and provides information about the thickness of the cornea. So the various stages of the disease can be clearly illustrated, especially in the case of keratoconus.
A corneal transplant is rarely required all of a sudden. As a rule, it is preceded by lengthy ophthalmology treatment, during which you will have spoken to your ophthalmologist many times about the disease itself and the treatment options, including corneal transplantation.
It is important to know that corneal transplantation requires a donor; corneal transplants always come from someone who has died.
There are several surgical methods available to the surgeon for the transplant itself:
In a penetrating (perforating) transplantation, a circular disc is cut out of the diseased cornea with the help of a special instrument. A disc of exactly the same size is then taken from the donor cornea and sewn into the opening created in the recipient's eye using very fine sutures. The sutures used to sew the graft into the existing cornea are made of nylon and are many times thinner than a hair.
The term “femto-keratoplasty” refers to cutting out of the diseased tissue and the graft using a laser. The major advantage of this is the possibility of creating interlocking incision profiles so that the donor cornea and the recipient bed are interlocked. This increases the healing area and the mechanical stability of the scar. The disadvantage of this surgical method is that the laser can only cut through clear tissue, which is why it cannot always be used.
If the pathological change is limited to certain layers of the cornea, it is possible to replace them alone where appropriate (and maintain the healthy layers). In this type of lamellar transplantation, a distinction is made between posterior transplantation (at the inner layer of the cornea) and anterior transplantation (at the outer layer of the cornea):
During inner layer transplantation, the defective endothelium is removed from the diseased eye along with the Descemet membrane (the thickest basal membrane in the human body and one of six layers of the cornea; at birth it is about three micrometres (μm) and in adulthood eight to ten micrometres (μm) thick) as a circular section and replaced with a matching lamella of healthy donor tissue. This donor lamella is rolled up and inserted into the eye through a small lateral incision. There it is unfolded and then pressed to the back of the cornea with an air bubble, to which it adheres, while the air bubble is absorbed in due course within a few days.
In outer layer transplantation, the opposite happens: here, the anterior corneal layers are separated from the Descemet membrane and the endothelium and replaced with donor tissue from the same layers. In this case, the graft must be attached with fine sutures (similar to perforating keratoplasty). Here, too, an air bubble is inserted into the eye at the end of the operation, which presses the recipient Descemet membrane onto the donor disc.
In the case of inner layer transplantation, you must remain on your back (including when using the toilet and eating) for the first 24 hours after surgery so that the air bubble in your eye can press on the graft and thus ensure its attachment. This also applies to an outer layer transplant, but less strictly.
It is essential to avoid rubbing the operated eye with any real force or pressing on it (even indirectly, for example, against a pillow or the like). Pain immediately after surgery is rare, but cannot be completely ruled out. Please always inform your specialist if you experience any pain and do not wait until the pain subsides on its own.
After the operation, a dressing is applied over the operated eye, which can usually be removed the next day. You must use eye drops from the first day after surgery. It is very important that you adhere exactly to the prescribed dose and are diligent about taking the eye drops, as this will largely determine whether corneal transplantation is successful. Eye drops are usually administered for many months (at least six months, often longer) – in some patients even for several years.
It is also important that you attend all follow-up appointments with your ophthalmologist. In the initial period after the procedure, your eyes will need to be checked more frequently at first and then at increasingly longer intervals.
The human cornea is the crystal-clear window at the front of the eye, through which you can see the coloured iris and the black pupil. At a microscopic level, it is made up of five layers, each of which has its own specific purpose; moreover, it is coated on its surface with a wafer-thin layer of liquid called the tear film.
As with all transplants, there is a risk of a so-called rejection reaction (i.e. the risk that the operated eye will not accept the donor cornea) as soon as the body’s immune system begins to fight the foreign tissue. This rejection reaction can lead to further clouding and loss of vision.
Most patients are able to read again by the time they are discharged from the eye clinic. After that, the cornea clears completely within one to two weeks and vision continues to improve during this time. Many patients have full vision again after around six weeks.
The inpatient aftercare consists of an inpatient hospital stay of approximately one week, as well as the administration of eye drops and possibly immunosuppressants (substances that reduce the functions of the immune system) in order to minimise the risk of rejection.